Mechanism of the partial oxidation of methane to synthesis gas over Pd

The partial oxidation of methane to synthesis gas has been studied in an isothermal continuous flow reactor operated with the phases in plug flow, using a silica-supported palladium catalyst. The reaction mechanism involves the sequential combustion and reforming of methane. The catalyst bed is not uniform in terms of the composition of the palladium phase. The implications for investigations using a pulse apparatus are discussed. Finally, large palladium crystallites readily grow carbon filaments. This revised version was published online in July 2006 with corrections to the Cover Date.     

天然气甲烷部分氧化制合成气的研究进展

甲烷部分氧化制合成气是高转化率、高选择性、高空速、低H2/CO、温和的放热反应,综述了近几年来甲烷部分氧化制合成气的催化剂、反应机理及活性中心的研究进展及反应中的存在问题。     

Partial oxidation of methane to synthesis gas over Rh-hexaaluminate-based catalysts

The partial oxidation of methane to synthesis gas over catalysts consisting of Rh supported on hexaaluminates (BaAl₁₂O₁₉, CaAl₁₂O₁₉ and SrAl₁₂O₁₉) was investigated at atmospheric pressure and high reactant dilution in order to compare their performances within the kinetic-controlling regime. Comparison with the results obtained over a commercial Rh/-Al₂O₃ system indicates that hexaaluminate catalysts are active and selective in this reaction. Despite of the higher surface area of the support, hexaaluminate-supported catalysts were found less stable, active and selective than an -Al₂O₃-supported catalyst.     

甲烷空气催化部分氧化制含氮合成气的研究

采用常规的浸渍法制备了镍基催化剂和经过镧改性的镍基催化剂,研究了甲烷催化部分氧化制备含氮合成气的催化功能,结果说明,镍含量在8％时催化活性达到最好,同时加入镧进行改性后催化剂的活性和选择性有所提高;该催化剂对甲烷空气催化部分氧化制合成气在常压下具有较高的转化率,随压力升高,转化率明显下降,并且积极严重,通过向体系加入H2O和CO2可以提高加压条件下甲烷的转化率并抑制催化剂积碳,还可以获得H2／CO接近2的合成气,满足合成液体燃料的要求。     

Partial oxidation of methane over platinum metal gauze

The partial oxidation of methane to synthesis gas has been studied over a platinum gauze catalyst. The experiments were carried out at atmospheric pressure with a single gauze in a quartz reactor heated in an electric furnace. The furnace temperature was varied in the range 200–900°C and the space time in the range 0.00021–0.00042 s. The feed consisted of a mixture of CH₄O₂Ar2110 and carbon oxides and water were the main products. Oxygen was only partly consumed and relatively small amounts of hydrogen were formed.     

Heat transport limitations and reaction scheme of partial oxidation of methane to synthesis gas over supported rhodium catalysts

The partial oxidation of methane to synthesis gas over supported Rh catalysts is investigated, paying particular attention to removing heat transport limitations and identifying the reaction conditions within the kinetic-controlling regime. The results obtained suggest that the reaction follows the sequence of total oxidation to CO₂ and H₂O, followed by reforming reactions to synthesis gas.     

The state of Rh during the partial oxidation of methane into synthesis gas

The partial oxidation of methane to synthesis gas over an - and a -supported Rh catalyst has been studied at atmospheric pressure using in situ DRIFTS between 823 and 973 K. A surface intermediate species with IR band at 2000 cm^-1, correlating with the CO formation, was observed during the partial oxidation. DRIFT spectra of adsorbed CO at 323 K were used to study the state of Rh during the partial oxidation. The state of Rh at 973 K is proposed to be a matrix of metallic rhodium with clusters of partially reduced oxide phase with isolated Rh⁺ atoms dispersed on the support. Rh oxide with Rh⁺ cations is the state of Rh during partial oxidation of methane at 823 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic partial oxidation of methane to synthesis gas in a ceramic membrane reactor

Methane has been selectively converted to synthesis gas using a two-zone fixed bed of a Ni/Al₂O₃ catalyst inside a modified ceramic membrane. The first zone of the reactor was surrounded by an impervious wall, and therefore behaved as a conventional fixed bed reactor. In the second zone, some of the reaction products could preferentially diffuse out of the reactor, which yielded higher than equilibrium methane conversions. The influence of the different operating conditions has been studied, and the performance of the membrane reactor has been compared to that of a fixed bed reactor. The membrane reactor has also been used at pressures above atmospheric (2 bar), with good conversions and selectivities.     

NiO/MgO整体式催化剂上甲烷部分氧化制备合成气

甲烷部分氧化制备合成气反应过程具有反应速率快、能耗低和H₂与CO物质的量比适用于合成甲醇及F-T合成等优点,是一种有希望替代传统水蒸汽重整的方法。研究在NiO/MgO蜂窝陶瓷整体式催化剂上的甲烷部分氧化过程,主要考察涂层载体、活性组分Ni含量、涂层载体前驱体、焙烧温度和还原温度对催化剂反应性能的影响。采用XRD、H₂-TPR和N2吸附等表征前驱体及其负载活性组分NiO后的晶相、还原特性和吸附性能。结果表明,采用浸渍法制备催化剂时,Mg(NO₃)₂为涂层载体MgO前驱体,在NiO负载质量分数20%、焙烧温度（500~600） ℃和还原温度750 ℃条件下制备的催化剂NiO/MgO-N性能较好,活性较稳定;以NiO/MgO-N为催化剂,在反应温度800 ℃、n(O₂)∶n(CH₄)＝0.5和空速9 723 h^-1条件下,CH₄转化率94.4%,H₂选择性99.9%,CO选择性92.9%。     

Primary reaction steps and active surface sites in the rhodium-catalyzed partial oxidation of methane to CO and H2

The nature of surface sites responsible for methane activation and CO_x formation on Rh catalysts for the partial oxidation of methane to syngas was investigated. The interaction of H₄ with Rh-black after oxidative and reductive pretreatments was studied applying (a) pulse experiments at reduced total pressure (10^–4 Pa) and 1013 K in the temporal-analysis-of-product (TAP) reactor and (b) in situ DRIFTS at 973 K. The saturation of the metal surface sites with oxygen was found to inhibit methane dissociation. Direct methane oxidation to CO₂ on the oxidized surface sites proposed earlier was excluded. Methane is first dissociated on reduced surface sites; the carbon species formed, then, react with surface oxygen to CO₂. Rh sites responsible for methane activation are neither related to the formation of the Rh₂O₃ nor Rh⁰. Probably the partially oxidized species (Rh⁺) or highly dispersed Rh³⁺ entities act as active surface centers for the dissociation of methane. For supported catalyst, such sites are stabilized by the support, which on the other side acts as a source of active oxygen involved in the oxidation of surface carbon and hydrogen.     

Partial oxidation of methane to synthesis gas via the direct reaction scheme over Ru/TiO2 catalyst

The partial oxidation of methane to synthesis gas has been investigated over various supported metal catalysts. The effects of operational variables on mass and heat transport resistances were investigated for defining the kinetic regime. It is observed that, in the absence of significant mass and heat transfer resistances, high selectivity (up to 65%) to synthesis gas is obtained over Ru/TiO₂ catalysts in the low methane conversion range ( ) whereas only negligibly small selectivity to synthesis gas is observed over all other catalysts investigated under similar conditions. This indicates that the Ru/TiO₂ catalyst possesses unique properties, offering high selectivity to synthesis gas formation via the direct reaction scheme, whereas the other catalysts promote the sequence of total oxidation of methane to CO₂ and H₂O, followed by reforming reactions to synthesis gas. An increase of selectivity to synthesis gas, in the presence of oxygen, is achieved over the Ru/TiO₂ catalyst by multi-feeding oxygen, which is attributed to suppression of deep oxidation of H₂ and CO.     

国内外甲烷催化部分氧化技术进展

介绍了国内外甲烷催化部分氧化制合成气技术的研究进展。主要工艺包括固定床工艺、流化床工艺和陶瓷膜工艺。此外，在反应器方面，还对两项最新专利技术进行了讨论。最后，提出了甲烷催化部分氧化制合成气技术的发展趋势，为我国在天然气利用方面的技术开发提供了参考和依据。     

Partial oxidation of methane to synthesis gas. Behaviour of different Ni supported catalysts

The behaviour of Ni supported catalysts, obtained using Ni(NO₃)₂ and Ni-acetylacetonate as precursor compounds, is analyzed. It is observed that initial activities and selectivities are similar for both systems, but the stability differs significantly. The systems show different carbon structures and sintering rates, depending on the precursor compound employed.     

天然气部分氧化制合成气的研究和实践

叙述了天然气部分氧化法制合成气的原理、国内外的开发和生产情况。并用模拟法进行了部分氧化工艺优化条件的研究，提出了这一方法的使用范围。     

Effect of support on the conversion of methane to synthesis gas over supported iridium catalysts

A partial oxidation of methane was carried out using iridium catalysts supported on several metal oxides. The productivity of the synthesis gas from methane was strongly affected by the choice of support oxides for the catalysts. The synthesis gas production proceeded basically via a two-step reaction consisting of methane combustion to give H₂O and CO₂, followed by the reforming of methane from CO₂ and steam. Although the combustion and the reforming of methane from steam did not depend upon the catalyst support, a large variation in the catalytic activity for the reforming of methane from CO₂ was observed over Ir catalysts with different supports. The support activity order in the reforming of methane from CO₂ with iridium catalysts was as follows: TiO₂≧ZrO₂≧Y₂O₃>La₂O₃>MgO≧Al₂O₃>SiO₂. The same order was observed in the synthesis gas production from the partial oxidation of methane. This revised version was published online in August 2006 with corrections to the Cover Date.     

A dual catalyst bed for the autothermal partial oxidation of methane to synthesis gas

Dual bed catalysts were found to produce high yields (>85%) of hydrogen from methane and air in a millisecond contact time reactor. The dual bed catalyst consisted of a 5 mm platinum combustion catalyst followed by a 5 mm nickel steam reforming catalyst. The platinum catalyst was used to totally oxidize approximately one-quarter of the methane feed to carbon dioxide and water. In the nickel catalyst, the carbon dioxide and water reformed the remaining methane to hydrogen and carbon monoxide. This process is favored at high flow rates, because the heat generated in the platinum catalyst is convected to the nickel catalyst at a higher rate. The heat delivered to the nickel catalyst favors the endothermic reforming reactions that generate the hydrogen and carbon monoxide.     

Partial oxidation of methane to synthesis gas over some titanates based perovskite oxides

Ca_1–x - _x Sr _x TiO₃-based mixed oxide catalysts containing chromium, iron, cobalt or nickel were prepared and used in the oxidation of methane. The catalyst containing cobalt or nickel showed high activity for the synthesis gas production from methane. In the case of nickel containing catalyst, nickel oxide originally separated from the perovskite structure was easily reduced to nickel metal, which showed synthesis gas production activity. In the case of the cobalt containing catalyst, pretreatment with methane was required for high activity. Reduced metallic cobalt was formed from the perovskite structure, which revealed relatively high selectivity for the oxidative coupling of methane, and afforded synthesis gas production. Both the catalysts also catalyzed carbon dioxide reforming of methane and especially both high activity and selectivity were observed over the nickel containing catalyst.     

Effect of oxygen mobility in solid catalyst on transient regimes of catalytic reaction of methane partial oxidation at short contact times

Mathematical modeling of the effect of the oxygen mobility in a solid oxide catalyst on the dynamics of transients of fast catalytic reactions has been carried out. The analysis was based upon the redox mechanistic scheme with a due regard for diffusion of oxygen from the bulk of catalyst to its surface. Parameters of kinetic and mathematical models were selected via fitting of the experimental data for methane selective oxidation into syngas on 1.4%Pt/Gd_0.2Ce_0.4Zr_0.4O _x catalyst. The range of the Thiele parameter (φ) where the oxygen bulk diffusion affects the most strongly reaction transients corresponds to φε [0.3 ÷ 7]. For high-surface-area oxide catalysts, the bulk oxygen diffusion coefficients corresponding to this range of the Thiele parameter are in the range of 10^–18 ÷ 10^–13cm²/s.     

Failure mechanisms of ceramic membrane reactors in partial oxidation of methane to synthesis gas

In the course of generating synthesis gas (H₂, CO) from methane, we have observed two types of fractures occurring on the Sr(Co, Fe)O_x-type oxygen membrane reactors. The first type occurred shortly after the reaction started and the second type often occurred days after the reaction. To determine the causes of these fractures, we have examined the starting material and fractured membranes using a combination of X-ray diffraction and thermogravimetric analyses. We found that the first type of fracture was the consequence of an oxygen gradient in the membrane, pointing from the reaction side to the air side. This causes a lattice mismatch inside the membrane, leading to fracture. The second type of fracture, however, was the result of a chemical decomposition. We found that the Sr(Co, Fe)O_x-type membrane had been reduced to SrCO₃, and elemental Co and Fe by the synthesis gas generated in the reaction. The decomposition causes enormous expansion leading to a large crack along the axis of tube.     

甲烷液相部分氧化制甲醇中溶剂介质的研究进展

甲烷液相转化具有反应条件温和、能耗低、投资少等优点,研究甲烷液相部分氧化制甲醇对实现天然气的直接转化和利用具有极为重要的战略意义。本文介绍了甲烷液相部分氧化制甲醇反应中有关反应溶剂介质的研究进展,详细叙述了各种酸性介质、水以及乙腈溶剂在甲烷部分氧化制甲醇中的应用,总结了不同溶剂介质下的反应机理、催化剂、溶剂浓度等对甲烷转化的影响,探讨了溶剂在甲烷液相部分氧化中的作用,指出依据溶剂介质的性质和作用,开发环境友好、反应条件温和、转化效率高的优良新型溶剂是甲烷液相部分氧化制甲醇的重要研究方向。